Influence of ion-to-electron temperature ratio on tearing instability and resulting subion-scale turbulence in a low-$\beta_e$ collisionless plasma

Abstract

A two-field gyrofluid model including ion finite Larmor radius (FLR) corrections, magnetic fluctuations along the ambient field and electron inertia is used to study two-dimensional reconnection in a low $\beta_e$ collisionless plasma, in a plane perpendicular to the ambient field. Both moderate and large values of the ion-to-electron temperature ratio $\tau$ are considered. The linear growth rate of the tearing instability is computed for various values of $\tau$, confirming the convergence to reduced electron magnetodynamics (REMHD) predictions in the large $\tau$ limit. Comparisons with analytical estimates in several limit cases are also presented. The nonlinear dynamics leads to a fully-developed turbulent regime that appears to be sensitive to the value of the parameter $\tau$. For $\tau = 100$, strong large-scale velocity shears trigger Kelvin-Helmholtz instability, leading to the propagation of the turbulence through the separatrices, together with the formation of eddies of size of the order of the electron skin depth. In the $\tau = 1$ regime, the vortices are significantly smaller and their accurate description requires that electron FLR effects be taken into account.